Pump lasers have gained widespread acceptance and have become an indispensable component for use in modem fiber amplifier communication systems. As is well known to those skilled in the art, the pump laser generates pump light which is transmitted to a fiber amplifier. The fiber amplifier absorbs the pump light energy, and uses the absorbed energy to amplify an input signal that is then transmitted over an optical line to an intended site.
One particular pump laser which is widely used is a 980 nanometer (nm) pump laser, due to its high electrical-to-optical conversion efficiency and the low noise figure of its amplifier. The pump laser module typically includes a pump laser chip, coupling optics and/or fiber grating. The coating for the laser facets and the optical elements is typically designed for 980 nm wavelengths, to substantially prevent, if not eliminate, reflection of those wavelengths.
One amplifier used in these pump lasers is an Erbium-Doped Fiber Amplifier (EDFA), which typically has a signal wavelength of between 1530 to 1610 nm. Thus, the coating used for the optical elements is often not suited for these wavelengths, resulting in a high reflection of the signal wavelengths within the pump laser module. In addition, the EDFA is often operated in very high gain conditions, such that it generates a considerable amount of amplified spontaneous emissions (ASE) in the wavelength band (1530-1610 nm). Because the coating is not suited for those wavelengths, the ASE can leak through the wavelength-division-multiplexing (WDM) device and is reflected back to the EDFA by the laser pump module. The reflected ASE power causes a degradation of the noise figure for the EDFA.
In order to overcome this problem, others have used a WDM device with a high degree of isolation from the common port to the pump module port at the signal wavelength. While such an arrangement has proven somewhat effective, a less stringent alternative would be an improvement.
Therefore, the need exists for a system which improves the return loss within the signal band of the EDFA, and which is less stringent than prior solutions.